1. Field of the Invention
The present invention relates to a process for preparing an electroluminescent device of a compound semiconductor and, more particularly, to a process for preparing an electroluminescent device of a compound semiconductor by molecular beam epitaxial growth.
2. Description of the Prior Art
In preparing electroluminescent devices of compound semiconductors, molecular beam epitaxial growing method (MBE method) of excellent film thickness controllability and mass production efficiency is used for epitaxial crystal growth of a ZnS compound semiconductor on a single crystal semiconductor substrate, followed by formation of an electrode. For the crystal growth of ZnS by the MBE method, a single element of Zn and a single element of S or sulfur hydride (H.sub.2 S) are heated in Knudsen cells independently of each other and respective molecular (atomic) beams are supplied on a fully heated single crystal semiconductor substrate to effect crystal growth. Further, the electrode is formed by depositing an electrode material on the epitaxially grown ZnS film, placing them on a heated specimen table and applying a heat treatment.
Layers of multi-layer epitaxial growth crystals for semiconductor electroluminescent devices are doped with impurities for controlling the conduction type and luminescence color thereof. For example, the impurity elements usable herein include when ZnS is to be made as n-type, aluminum (Al), gallium (Ga), indium (In), group VII elements such as iodine (I), bromine (Br), chlorine (Cl) and fluorine (F) and, when it is to be made as p-type, group I elements such as lithium (Li), sodium (Na) and potassium (K), and group V elements such as nitrogen (N), phosphorus (P), arsenic (As) and antimony (Sb). When it is to be made semi-insulating, dopants are not added, or group IV element such as silicon (Si) or germanium (Ge) and a combination of one of the above-mentioned groups III and VII elements and one of the above-mentioned groups I and V elements are used. Another doping is often conducted, particularly, for providing luminescent centers and, in this instance, manganese (Mn) and Lanthanoids (rare-earth elements) including lanthanum (La), cerium (Ce), praseodymium (Pr), promethium (Pm),, samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutenium (Lu) can be used as impurities.
An epitaxial film according to the MBE method is formed by irradiating a substrate heated to a suitable temperature with a molecular beam or with such a beam as heated to a high temperature and also by irradiating with a molecular beam for doping elements at the same time.
Further, as a steps from the deposition of the electrode material for forming an electroluminescent device to the heat treatment, In-Hg alloy material is usually deposited on a ZnS epitaxial layer and, subsequently, an electrode (ohmic contact) is developed by heating to 350.degree. C.-450.degree. C. in an atmosphere such as H.sub.2 --Ar gas mixture (refer to: "Growth of ZnS Bulk Single Crystal and Homoepitaxial Growth of ZnS by Molecular Beam Epitaxy", "Extended Abstracts of the 19th Conference on Solid State Devices and Materials", Tokyo, 1987, pp. 247-250; "SINGLE CRYSTAL GROWTH OF ZnS BY THE METHOD OF GAS SOURCE MBE," Journal of Crystal Growth, 76(1987)440-448, North-Holland, Amsterdam; "Fundamental Issues in Heteroepitaxy" J. Mater. Res., Vol. 5, No. 4, pp. 852-894, Apr 1990).
However, many of starting elemental materials including the impurity elements described above (molecular beam materials, e.g., zinc, sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, iodine, bromine, chlorine and fluorine) have high vapor pressure and low adhesiveness on the substrate which is heated to a suitable temperature required for the growth of compound semiconductors (at least 300.degree. C.), and therefore, it is impossible to grow high-quality single crystals of the compound semiconductors.
On the other hand, if compound semiconductors are grown at a high temperature which permitting the growth of high-quality crystals of each of the constituent compounds, they will develop point defects such as vacancies leading to deep levels or complex defects and become contaminated with objectionable impurities, thus resulting in serious drawbacks in respect of the characteristics of the compound semiconductors and it is, accordingly, desired to grow them at the lowest possible temperature.
However, of the impurity elements or constituent elements of semiconductors, the metal elements (Zinc, cadmium, aluminum, gallium, indium, sodium, potassium, silicon, germanium, manganese and all the lanthanoid elements) are present in the form of mono-atomic molecules, whereas they are liable to aggregate on a substrate at low temperature to form solids without forming a compound or without diffusing as impurity elements. Further, the non-metallic elements are present usually in the form of diatomic molecules (tellurium, nitrogen, iodine, bromine and fluorine), tetraatomic molecules (arsenic and phosphorus) or polyatomic molecules containing two to eight atoms (sulfur and selenium). Accordingly, during the growth on the substrate at low temperature, decomposition and incorporation of them into the crystals fail to proceed smoothly, causing structural defects (minute twin crystals, crystal grain boundary of small tilt angle, minute island-like growths, etc.), making it extremely difficult to grow single crystals at high-quality and with controlled doping usable for the fabrication of semiconductor devices. Accordingly, the semiconductor multi-layer epitaxial crystals prepared by the conventional growing method, even when doped with impurity elements in a controlled fashion, become degraded when heated to a temperature not lower than the growth temperature. For example, it is not exceptional that a low-resistivity semiconductor epitaxial film fully doped with impurities increases in its resistivity by at least 10 orders of magnitude when treated by heating. Further, because they increase the resistivity when heated again up to the vicinity of the growth temperature, formation of the electrode involves a problem that additional ingenuity must be exercised in forming the electrode layer to avoid such a heat treatment. There is another problem of remarkable deterioration of the electroluminescent device after driving.
Incidentally, although MOCVD as conducted under the irradiation with light is known (Sg. Fujita, A. Tanabe, T. Sakamoto, M. Isemura and Sz. Fujita, Jpn. J. Appl. Phys., 26 (1987) L2000; Sz. Fujita, E. Y. Takeuchi and Sg. Fujita, Jpn. J. Appl. Phys. 27 (1988) L2019), application of light for MBE is not known.